The inclusion of Radar Absorbing Material (RAM) into composite blades of wind turbines to reduce its radar reflectivity so that they do not interfere with radar systems such as air traffic control systems, weather or marine radar systems is known.
WO 2010/122350 A1 describes a combined prepreg material comprising first and second layers impregnated with a matrix material such as a resin to be used particularly in wind turbine blade. The first layer is a functional layer of RAM and the second layer is a keying layer that comprises a keying medium to facilitate bonding of the combined prepreg material to a gel coat.
U.S. 2012/0207612 A1 discloses a wind turbine blade that includes a multilayer composite structure including a first reflective layer and a second layer with a plurality of resistive circuit analogue (CA) elements. The CA elements are tuned so as to interact with said first layer to provide absorption of electromagnetic (EM) energy over a desired frequency range. The parameters of the CA elements can be varied to provide for frequency tuning and to maintain absorption at a specific frequency range despite varying layer separation, while at the same time ensuring that the mechanical properties of the CA layer are compatible with integration into the turbine blade.
WO 2015/061670 A1 discloses a suitable composite laminate for wind turbine blades for the attenuation of the reflection of electromagnetic radiation in the S or X bands up to a peak of −20 dB. The composite laminate comprises particularly one or more functional layers having a printed circuit for absorbing the electromagnetic radiation incident on the composite laminate and a conducting layer.
None of said patent applications mentions any relation between RAM and lightning protection.
U.S. 2014/0118177 A1 discloses a wind turbine blade wherein the RAM includes a ground plane having an electrical conductivity and/or a dielectric constant that is higher in the presence of an electric field having a frequency of 1 GHz and above than in the presence of an electric field having a frequency of 10 MHz and below. The ground plane can be tuned to be highly reflective at radar frequencies and benign at lightning discharge frequencies.